The subject matter of the present invention pertains to digital first motion detection associated with compressional waves in array sonic data, and more particularly, to a method and apparatus for generating threshold crossings from each analog waveform generated by a receiver in an array sonic service tool and for determining the first arrival time of a compressional wave at each such receiver thereby determining a borehole formation property adjacent a plurality of the receivers.
Well tools are adapted to be lowered into boreholes for the purpose of generating data representative of the characteristics of an earth formation in the borehole. One such well tool is a sonic tool, a tool that pulses the formation with dipole and/or monopole "pressure waves" thereby inducing shear waves and compressional waves in the formation, the shear and compressional waves propagating up the formation from the tool pulse source transmitter to a plurality of tool receivers. If one could sense the first arrival time of a compressional wave at each receiver, a time difference "delta-T" could be determined associated with the propagation time of the compressional wave between adjacent receivers, the delta-T being representative of the properties of the formation (e.g., compressional wave slowness) between the adjacent receivers One prior art sonic tool owned by the assignee of this patent is called the "Array Sonic Service" tool, or the "Sonic Digitizing Tool" (SDT). The SDT tool operated in two modes: the digital mode and the analog mode. In both modes, the SDT attempted to determine the first arrival time of compressional waves at each receiver almost entirely within the confines of the well logging truck computer located at the well surface. During the digital mode, analog data, representing first arrival time data, was first digitized and the digitized analog data was transmitted uphole to a well truck computer. The digitized analog data was processed entirely by the well logging truck computer software. From a design standpoint, it is preferable to design a telemetry system utilizing digital circuitry as opposed to analog circuitry. Although the analog data was digitized, the quantity of such digitized analog data required complicated and expensive digital telemetry circuits for transmitting the digitized analog data uphole. During the analog mode, the analog data was transmitted uphole to the well truck computer; however, it was contaminated with ambient noise during its transmission uphole. It was necessary to account for the presence of this noise during the processing by the truck computer. In addition, the SDT was limited to one detection per threshold; using this implementation, a detection on noise results in an incorrect travel time reading. A more desirable approach to compressional wave first motion detection is to first generate, within the well tool, a reduced or compressed quantity of data for transmission uphole, relative to the digitized analog data generated by the SDT tool. The compressed data should be free of ambient noise. In the context of this discussion, this compressed quantity of data is termed digital threshold crossing or zero crossing data. Using this more desirable approach, the digital threshold crossing data would be transmitted uphole, via a digital telemetry system, to a well logging truck computer; and, using a special first motion detection software embodied in the truck computer memory, a set of first arrival times would be determined, associated with a compressional wave propagating in a surrounding formation, a first arrival time being determined for each receiver. The first arrival times, for each receiver, further determine compressional wave slowness between adjacent receivers in the surrounding formation and ultimately the characteristics or properties of the surrounding earth formation in the vicinity of the receivers within the borehole.